As a ground work for the study of the relationship between the amino acid sequence and the stabilizing energy of the three dimensional structure, the X-ray diffraction of crystals of Nuclease-T bound with ligands has been measured at 2 to 4 degrees C to resolution between 2 and 3A using a precession camera in order to determine the atomic coordinates. On the basis of measurements using a stopped flow technique folding of nuclease A portion of nuclease B appears to be thermodynamically and kinetically independent of the 19 residues at the NH2-terminus. The equilibrium constant of Nuclease-(1-126) with deoxythymidine 3',5'-diphosphate in the presence of O.01M Ca 2 ion has been measured by a dialysis equilibrium method.